Demand for lithium secondary batteries as energy sources is rapidly increasing as mobile device technology continues to develop and demand therefor continues to increase. Recently, use of lithium secondary batteries as a power source of electric vehicles (EVs) and hybrid electric vehicles (HEVs) has been realized. Accordingly, research into secondary batteries, which may meet a variety of requirements, is being actively performed. In particular, there is high demand for lithium secondary batteries having high energy density, high discharge voltage, and output stability.
In particular, lithium secondary batteries used in hybrid electric vehicles must exhibit great output in a short time and be used for 10 years or more under harsh conditions of repeated charge and discharge on a daily basis. Therefore, a lithium secondary battery exhibiting superior stability and output characteristics to existing small-sized lithium secondary batteries is required.
In connection with this, existing lithium secondary batteries generally use a lithium cobalt composite oxide having a layered structure, as a cathode and a graphite-based material as an anode. However, LiCoO2 has advantages such as superior energy density and high-temperature characteristics while having disadvantages such as poor output characteristics. Due to such disadvantages, high output temporarily required at abrupt driving and rapid accelerating is provided from a battery and thus LiCoO2 is not suitable for use in hybrid electric vehicles (HEV) which require high output. In addition, due to characteristics of a method of preparing LiNiO2, it is difficult to apply LiNiO2 to actual production processes at reasonable cost. Furthermore, lithium manganese oxides such as LiMnO2, LiMn2O4, and the like exhibit drawbacks such as poor cycle characteristics and the like.
Accordingly, a method of using a lithium transition metal phosphate as a cathode active material is under study. The lithium transition metal phosphate is widely classified into LixM2(PO4)3 having a NaSICON structure and LiMPO4 having an olivine structure, and considered as a material having superior stability, when compared with existing LiCoO2.
A carbon-based active material is mainly used as an anode active material. The carbon-based active material has a very low discharge potential of approximately −3 V, and exhibits extremely reversible charge/discharge behavior due to uniaxial orientation of a graphene layer, thereby exhibiting superior electrode cycle life.
Meanwhile, lithium secondary batteries are prepared by disposing a porous polymer separator between an anode and a cathode, and inserting a non-aqueous electrolyte containing a lithium salt such as LiPF6 and the like thereinto. Lithium ions of a cathode active material are released and inserted into a carbon layer of an anode during charging, whereas lithium ions of the carbon layer are released and inserted into a cathode active material during discharging. In this regard, a non-aqueous electrolyte between an anode and a cathode functions as a medium in which lithium ions migrate. Such lithium secondary batteries must be basically stable in a range of battery operation voltage and must have ability to transfer ions at a sufficiently fast speed.
As the non-aqueous electrolyte, existing carbonate based solvents were used. However, carbonate based solvents have problems such as decreased ionic conductivity due to increased viscosity.
Therefore, there is an urgent need for technology to resolve the problems.